The invention relates to a transparent, flame-retardant, thermoformable film made from crystallizable thermoplastics, the thickness of which is preferably in the range from 10 to 350 xcexcm. The film comprises at least one flame retardant and has very good optical and mechanical properties, and also has good orientability and, respectively, thermoformability, and can be produced cost-effectively. The invention further relates to the use of this film and to a process for its production.
Transparent films made from crystallizable thermoplastics with a thickness of from 10 to 350 xcexcm are well known.
These films are not flame-retardant and not thermoformable, and therefore neither the films nor the items or moldings produced from them are suitable for applications where thermoformability and fire protection or flame retardancy are demanded.
DE-A 2346 787 describes a phospholane-modified flame-retardant polymer. As well as the polymer, the use of the polymer for producing films and fibers is claimed.
The following shortcomings are apparent when this phospholane-modified polymer is used for film production:
The polymer is very susceptible to hydrolysis and has to be very thoroughly predried. When the polymer is dried using dryers of the prior art it cakes, making production of a film impossible except under very difficult conditions.
The resultant films produced under uneconomic conditions embrittle when exposed to heat, i.e. the mechanical properties deteriorate due to embrittlement, making the film unusable. This embrittlement occurs after as little as 48 hours of exposure to heat.
It is an object of the present invention to provide a transparent, flame-retardant, thermoformable oriented film with a thickness of preferably from 10 to 350 xcexcm which is not only cost-effective to produce and has good orientability and good mechanical and optical properties, but in particular is thermoformable and flame-retardant, and does not embrittle after exposure to heat.
Thermoformability means that the film can be thermoformed to give complex and large-surface-area moldings on commercially available thermoforming machinery without uneconomic predrying.
Flame retardancy means that in a fire protection test the transparent film fulfils the conditions of DIN 4102 Part 2 and in particular the conditions of DIN 4102 Part 1, and can be allocated to construction materials class B2 and in particular B1 for low-flammability materials.
The film is also intended to pass the UL 94 test (Vertical Burning Test for Flammability of Plastic Material), and therefore to be capable of classification to class 94 VTM-0. This means that 10 seconds after removal of the Bunsen burner the film has ceased to burn, after 30 seconds no glowing is observed, and also no dripping occurs.
Examples of the good optical properties are high light transmittance ( greater than 80%), high surface gloss ( greater than 100), low haze ( less than 20%), and also low Yellowness Index (YI less than 10).
Examples of the good mechanical properties are a high modulus of elasticity (EMD greater than 3200 N/mm2; ETD greater than 3500 N/mm2), and also good values for tensile stress at break (in MD greater than 100 N/mm2; in TD greater than 130 N/mm2).
Good orientability includes excellent capability of the film to be oriented, without break-offs, both longitudinally and transversely during its production.
Cost-effective production includes the capability of the raw materials or the raw material components needed for producing the flame-retardant film to be dried using commercially available industrial dryers, such as vacuum dryers, fluidized-bed dryers, or fixed-bed dryers (tower dryers). It is important that the raw materials do not cake or become thermally degraded. The dryers mentioned operate at temperatures from 100 to 170xc2x0 C., at which the flame-retardant polymers mentioned cake, and the dryers or extruders become blocked so that the carbonized mass has to be dug out, making film production impossible.
The vacuum dryer, which has the gentlest drying action, passes the raw material through a range of temperatures from about 30 to 130xc2x0 C. in a vacuum of 50 mbar. What is known as post-drying is then required in a hopper at temperatures of from 100 to 130xc2x0 C. with a residence time of from 3 to 6 hours. Even here, the flame-retardant polymers prepared according to the prior art cake to an extreme extent.
No embrittlement on exposure to heat means that after 100 hours of a heat-conditioning procedure at 100xc2x0 C. in a circulating-air oven the film has not embrittled nor has disadvantageous mechanical properties.
The object of the invention is achieved by means of a transparent oriented film with a thickness of preferably from 10 to 350 xcexcm which comprises, as main constituent, a crystallizable thermoplastic, wherein this film comprises at least one flame retardant. This film is a mono- or biaxially oriented.